We plan to investigate several iron proteins from two bacterial enzyme systems, a monoxygenase and a cytochrome oxidase. The iron in these proteins is involved in O2 binding and activation and/or electron transfer; from an analysis of its electronic state the function of the active center can be clarified. Mossbauer spectroscopy, our main tool, measures electric and magnetic hyperfine interactions of the 57Fe nucleus with the iron valence electrons and characterizes the electronic ground state in detail. Comparison of data obtained from various states allows one to trace the changes taking place at the active center and helps to elucidate the reaction mechanism. Specifically we will study: 1. Cytochrome P450 from the camphor hydroxylase of Pseudomonas putida. We plan to extend earlier high-field Mossbauer measurements on the reduced protein to higher temperatures and to complement data on frozen solutions of the oxidized protein by single crystal ESR experiments and Mossbauer measurements on polycrystalline samples. 2. Cytochrome oxidase/nitrite reductase from Pseudomonas aeruginosa, a dimer containing two heme c and heme d groups, can catalyze the four-electron reduction of O2 and is therefore a functional analog of the terminal oxidase of the mitochondrial respiratory chain. Cytochrome oxidase and two other proteins of the bacterial electron transfer chain, cytochrome c551 and azurin, have been crystallized. The hemes can be selectively enriched in 57Fe and the three proteins together provide a well defined system for a Mossbauer study of electron transfer, complex formation and O2 reduction.